Chronic diarrhea is the most prevalent clinical disease entity in nonhumanprimate colonies worldwide. Often an etiologic agent is not found. In addition, many animals fail to respond to symptomatic therapy. In the past, the veterinary staff had found that many of these animals responded quickly to low dose tetracycline therapy. A study was undertaken to evaluate clinical response in animals with chronic idiopathic diarrhea using low dose tetracycline therapy. Prior to entrollment on the study, animals had demonstrated diarrhea for no less than 30 consecutive days, had three negative stool samples (flotation, giardia ELISA, cryptosporidial ELISA, sedimentation) and had three negative bacterial rectal swab cultures. Viral culture and antibody testing was performed for simian retrovirus (SRV). A complete blood count (CBC) serum chemistry and colonic endoscopy with biopsy were also performed prior to inclusion on the study. Monitoring during the study period was fulfilled by physical examination, CBC, serum chemistry, bacterial rectal swab culture, fecal exam for parasites, and colonic endoscopy with biopsy. These disagnostics were repeated weekly during dosing, one week after discontinuation of the drug, and two months after discontinuation of the drug. Tetracycline was administered for 30 days once daily at 40 ml regardless of body weight. Eighteen Macaca mulatta from the Tulane Regional Primate Research Center breeding colony were used for the study. Animals ranged in age from 1.67 to 19 years. Sixty-seven percent (12/18) of the animals responded to tetracycline therapy by producing normal stools. Days of diarrhea prior to treatment ranged from 30 to 114. Relapse rate after therapy was withdrawn was 42%. Rate of response to therapy or relapse rate were not correlated with age or length of time of diarrhea prior to treatment. Physical examination revealed an increase in mean body weight and normalization of hydration status during treatment. Histopathology prior to and after tetracycline therapy revealed no significant changes within and comparing responders with non responders. All histo sections were described as normal. Future studies will involve pharmacokinetic evaluation of peripheral and intestinal